While it's sometimes easier to hope that the older dogs will teach the little one, that doesn't always happen. While I had it work with Apache, it didn't with Zuni.
When I was house training Zuni I would leash her up and take her out into the yard and just wait until she went, then praise and treat. There were many times she wouldn't move, so we just stood there until she moved and went or went where she was. It can be a real pain, especially in the middle of the night…but that doesn't last forever. Any accidents in the house were just cleaned up and not acknowledged.
When we lived in the city, the dogs did prefer to go while on walks, but would, if they REALLY needed to go, go out into the yard.
